1. Field of the Invention
The present invention relates to full duplex data transmission between two transceiver terminals. In particular, the present invention relates to data transmission utilizing a single optical fiber to connect a pair of transceivers wherein transmission from each transceiver is obtained by coupling a light signal to the single fiber with a specific type of directional coupler.
2. Description of the Prior Art
Conventional data transmission is normally achieved by utilizing metal wire transmission lines such as twisted wire pairs or coaxial cables. The lines are connected between two terminals at either end of a data link. Such links are capable of carrying transmission in one direction only at a given point in time because of the nature of the propagation of electrical current. Bidirectional transmission, i.e. transmission in both directions over a single wire or data link, can only be approximated in wire transmission lines by the interruption of the transmission from a given terminal to allow transmission from the other terminal or by duplication of communication paths. This transmission on a time-sharing basis decreases the overall rate of information transfer by at least 50% as compared to continuous data transmission. In addition, timing for this time-sharing format is critical in order to achieve the proper send and receive synchronization and thus a considerable amount of electronic circuitry must be devoted to the timing and synchronization functions.
The prior art suggests several methods of transmitting and receiving optical communications in order to obtain continuous bidirectional transmission. For instance, light beam communication systems have been suggested for many years. In U.S. Pat. No. 2,100,348 of Nicolson and U.S. Pat. No. 3,351,761 of Hamby et al., transmission systems are described which utilize light, but not optical fibers. That is, the light is focused through lenses and transmitted through the atmosphere.
Sick in British patent specification No. 1,429,875 suggests an alternate method wherein a focusing system and dichroic mirrors are utilized. The dichroic mirrors are used as beam splitters and bend the light transmitted through the atmosphere. In another system Nussmeier in U.S. Pat. No. 3,654,473 suggests the use of a laser beam which is polarized and phase modulated with information. The two perpendicular components of the polarized laser signal are used for the bidirectional transmissions.
Wavelength multiplexing and frequency multiplexing in order to avoid the time-sharing problem also requires considerable circuitry.
Continuous bidirectional data transmission has been performed using microwave waveguides and directional couplers. In these systems signals from both transmitters are coupled simultaneously and in opposite directions into a transmission line waveguide by way of two directional couplers. The directional property of the couplers prevents mixing of the two signals at the receivers; consequently the receivers cannot receive from their related transmitters, but may only receive from the remote transmitter. Optical fibers, on the other hand, have waveguide characteristics in that they can transmit two signals in opposite directions without interference in a manner similar to microwave waveguides. However, optical fibers, as opposed to microwave waveguides, are not necessarily single-mode waveguides. Thus, optical fiber systems using directional couplers allow manufacturing of multimode devices. Also, the systems are capable of transmitting signals and wavelengths throughout the visible and near infrared spectra.
It has now been suggested that directional couplers be utilized to transmit television signals on a single optical fiber cable (see Sept. 14, 1978 issue of Electronics, page 70). The suggested system, however, utilizes standard directional couplers which have a 3 dB loss inherent in the splitting of the signals. These conventional couplers divide the light in half, with half of the signal from the remote transmitter being transmitted to the adjacent receiver, and the remaining half being transmitted to the associated receiver. Thus this type of system inherently loses half of its power.